Luminance homogeneity over the lighting area (or light emitting area) of an organic light emitting diode device (OLED device) has an impact on both, aesthetical perception and reliability of the device. Since luminance is proportional to current density, it is therefore crucial to have very high luminance homogeneity over the life time of the device. A specific aspect of OLED devices is that light is emitted all over the light emitting area.
A drawback involved herewith is that the device is very sensible to current inhomogeneities. The resistance of an organic stack is lower at high current density which forces most of the current flowing over the anode surface to pass through the low resistance area called “hot spot”. As a consequence the low resistance area will heat very fast and a runaway loop process will start between current and T° leading to a reduction in life time and possibly to damage and failure very early in the use of the device. This process is accelerated by stress on the device.
Consequently, it is desirable to prevent such weak spots from appearing and to keep the current homogeneity as good as possible along the life time of the device.
In practice, a small area with high local current density could appear easily due if the device is not cooled uniformly over the lighting area: for example, if the device is submitted to forced national convection from bottom to top in case of a vertical arrangement, in case of an inappropriate handling like using a material with different thermal properties over the area to hold the device or simply if better cooling is applied on a limited part of the part of device. A simple local input of energy (like, for example, a light reflection of a spot locally) on the lighting area of the device could also be the cause of hot spot.
Further, it is also desirable to design devices to achieve lower sensibility to mechanical, thermal or electric stresses. This could be done by appropriate substrate layout design and system architecture. Basically, a conventional solution consists in limiting the size of the lighting area and limiting the current stress to prevent the level where runaway loop between current and heat starts from being reached. A second possibility is to improve the current distribution via appropriate substrate layout design. In this case the basic principle is to reduce power losses over the highly resistive ITO anode. In practice, in order to reduce power losses, several solutions are applied by OLED manufacturers. Typical solutions are optimizations of the shape and dimension of electrode contact areas and also optimizations of the anode conductivity.
However, even after an optimization of the layout, the current distribution is never perfectly homogeneous and weak areas with high sensibility to level of external stress (current, temperature) are always generated.
Thus, there is a demand for a possibility of designing OLED devices with a reduced number of hot spots.
It was found that hot spots are usually situated in corners of the light emitting area. In order to address this, a known solution provides for cutting the corners of the light emitting area or for increasing the radius of the corners. A particular drawback of this approach is that the visual perception of the device is disturbed by design irregularities, while cutting the corners and/or increasing the corner radius limits the design options for using the devices.
US-2013/009199 discloses an organic light-emitting device having an active layer with first and second side surfaces adjoining a corner edge, wherein the first side surface has a recessed region adjoining the corner edge. For injecting charge carriers into the active layer, the device has a metallic contact layer extending along the first side surface of the active layer, and comprising a triangular structure so that the nearer it approaches the recessed region, the more it is recessed from the first side surface of the active layer, so that the injection of charge carriers from the metallic contact layer into the active layer is suppressed in the recessed region.